How to format 'nix eval' output with «» - double angle quotation marks

Hello, I am having difficulties in the way of debugging nix config. For nix eval, --json isn’t applicable for outputs that contain functions (error: cannot convert a function to JSON). So manually piping to nixfmt is an option, but nixfmt can’t handle «» ( guillemets/double angle quotation marks). I tried using a simple to string conversion sed --null-data -E 's/«([^»]*?)»/"«\1»"/g' | nixfmt, but then nested guillemets (««»») stop it. For example: nix eval github:nix-community/emacs-overlay#...